Chemistry: The Central Science (13th Edition)

Chapter 10 - Gases - Exercises: 10.62c

Answer

It will decrease

Work Step by Step

Mole fraction is (moles of component i / total moles). In the case of mole fraction of B, it would be the (moles of B/ total moles in container). Adding a third gas will increase the total moles in the container, but not the moles of B. This will lead to the denominator getting larger, but not the numerator, which makes the mole fraction smaller.